Printed circuit boards (PC boards) are widely used in electronic devices. Electronic circuitry associated with these devices is typically mounted on one or more PC boards. A PC board comprises a generally planar insulating base defining opposite surfaces on which a pattern of conductive traces is printed. The conductive traces are formed by etching, plating or stamping. The traces connect electronic components mounted on a surface or side of the insulating base.
To achieve a greater density of circuitry and better avoid internal interference on a PC board, electronic components may be mounted on both surfaces of the board. For example, PC boards used in radio communications devices may advantageously have radio frequency (RF) circuitry mounted on one surface of the board and digital processing circuitry mounted on the opposite surface. Conductive traces are printed on each surface of the board to provide necessary electrical connections for the mounted electronic components. Further, connections which traverse through the PC board are provided to permit communications of signals between components on opposite surfaces of the board.
In complex electronic devices, circuitry disposed on either side of a PC board may be comprised of several functional circuits, i.e. voltage regulation circuitry, signal amplification circuitry, frequency modulation circuitry, etc. These functional circuits are often grouped together and partitioned from other areas on the PC board by means of a printed ground traces. The ground traces provide a convenient connection to ground for nearby circuitry. If a connection between circuits in different sections of the board need to be made on a surface of the PC board, a suitably positioned gap is provided in the ground trace separating the circuits. These gaps allow one or more conductive traces to pass through the ground traces and connect the respective sections. One or more grounding plates are sandwiched between insulating layers of the base. Connections between the ground traces and the grounding plates are provided by vias, which are conductively plated througholes extending between the ground traces and the grounding plates.
Operation of electronic circuitry having a time varying signal generates electromagnetic radiation and results in electromagnetic fields emanating from the circuitry. An electromagnetic field generated by circuitry in one section of the PC board may be picked up by and interfere with the operation of circuitry in other sections of the board (or, if the electromagnetic field is strong enough, circuitry on adjacent PC boards). Undesirable electromagnetic fields generated by energized circuitry are commonly referred to as electromagnetic interference (EMI).
In the case of radio communications equipment, unwanted RF noise or leakage emitted by equipment circuitry may escape from the equipment and interfere with the operation of other nearby electronic devices. FCC regulations limit the quantity of RF leakage which is permitted to escape into the environment from various classes of radio communications equipment.
One proposal advanced to address the aforementioned EMI and RF noise problems is illustrated in FIG. 1. As illustrated, this proposal utilizes a metal fence and lid assembly, shown generally at 10, which overlies and is secured to a PC board 12. The PC board 12 includes a series of ground traces 14 on a surface 15 of the board. The assembly 10 is comprised of an upright fence 16 which is constructed from thin strips of metal. The metal strips are bent and spot welded to adjacent metal strips to form a shape congruent with the ground traces 14. The upright fence 16 is comprised of an outer fence 20 and inner fence segments 22. The outer fence 20 is formed by bending one or more metal strips into a rectangular-shaped fence corresponding to the shape of an outer periphery of the ground traces 14 and spot welding overlapping ends of the strip together. The inner fence segments 22 have short right angled portions 24 at each end. The right angle portions 24 are spot welded to the outer fence 20 or to another inner fence segment 22. The inner fence segments 22 are sized and positioned to congruently overlie the ground traces 14 on an interior portion of the PC board.
Following assembly of the upright fence 16, the fence is positioned on the PC board 12 so as to overlie the ground traces 14. The fence 16 is soldered to the traces 14. Subsequently an inner lid 26 is soldered to a top edge 28 of the fence 16. The inner lid 26 and the fence 16 reduce EMI between circuitry disposed on different sections of the PC board 12.
Finally, a flat metal outer lid 30 is positioned on top of the entire upright fence 16. The outer lid 30 is soldered to the upright fence 16 at four corners of the outer fence 20. The lid 30 and the outer fence 20 reduce RF leakage from the energized circuitry on the PC board. Metal components are used due their ability to contain electromagnetic radiation.
Unfortunately, the aforementioned proposal has numerous disadvantages. For example, the assembly of the upright fence 16 includes spot welding strips of metal together and often results in uneven fence heights. Thus, when the flat inner and outer lids 26, 30 are secured to the fence, there may be gaps between the lids and the fence permitting electromagnetic interference between circuits and/or RF signal leakage into the environment. Additionally, the process of soldering the upright fence 16 to the ground traces 14 can also be problematic. The ground traces 14 are typically only on the order of 0.05 inches wide, thus, it is easy to accidentally ground out adjacent conductive traces and/or components while soldering the upright fence 16 to the ground traces 14. Once constructed, repair or modification of the circuitry on the PC board 12 is difficult to accomplish because of the upright fence 16 and lids 26, 30 being soldered. Additionally, assembly of the fence and lid assembly 10 is painstaking and constitutes a significant cost in time and money which increases the price of the device within which the PC board is to be utilized.
Another proposal for controlling RF leakage involves inserting the PC board into a plastic bag coated on both sides with a thin metal layer. The opening of the plastic bag is sealed using tape or another fastening means. This method of controlling RF leakage has proven only marginally efficient. Additionally, this method does not remedy the problem of electromagnetic interference between circuits located on different sections of the board. Therefore, interior fences must be constructed and soldered to the ground traces to isolate circuit sections. Thus, this proposal does not provide a complete solution to the electromagnetic interference and RF signal leakage problems.